1. Field of the Invention
The present invention relates to a developing agent density control method of controlling a toner-to-carrier ratio for use in an image forming apparatus which develops a latent image formed on an image carrier by using a two-component developing agent consisting of a toner and a carrier.
2. Description of the Related Art
FIG. 15 illustrates an image forming apparatus which relates to the present invention.
In the image forming apparatus shown in FIG. 15, a photosensitive drum 1 which is rotatably supported to rotate in a direction indicated by an arrow is evenly charged by a primary charger 2. Subsequently, a color-separated light image 3 is radiated thereon to form an electrostatic latent image on the photosensitive drum 1.
The electrostatic latent image formed on the photosensitive drum 1 is then developed into a toner image by moving a predetermined one of a plurality of developing units 5 (5M, 5C, 5Y, and 5B), which are mounted on a moving table 7 and conveyed in the direction of a tangent line of the photosensitive drum 1, to a developing section, i.e., a developing position 4, and by applying a development bias from a development bias voltage source 6.
The resultant toner image on the photosensitive drum 1 is transferred by a transfer charger 9 onto a transfer material which is supplied from a transfer material cassette 20 to a transfer drum 8.
While the transfer charger 9 is OFF, a patch image with a predetermined density is formed on the photosensitive drum 1 by the same development bias as that during printing in order to perform density sensing. As shown in FIG. 16, a sensor constituted by a light-emitting element 51, an optical transparent window 52, a light-receiving element 53, and a light-receiving element 54 for monitoring direct light from the light-emitting element 51 senses the density of the patch image, and a CPU 55 performs calculations in accordance with the sensed density. On the basis of the calculation result, the CPU 55 replenishes a developing unit with a toner if necessary, thereby performing control such that the image density is kept constant.
The toner image and the patch image not transferred but remaining on the photosensitive drum 1 are removed from the photosensitive drum 1 by a cleaning unit 10. The photosensitive drum 1 is then used in the formation of the next image.
In the apparatus of the above sort, the image density preferably varies little during printing due to a variation in the mixing ratio of the toner to the carrier.
For this reason, the developing agent density variation does not appear easily as the image density variation. Consequently, although the developing agent density control is performed, the developing agent density variation becomes large to result in unstable image formation.